Sit on it: Commish Janette Sadik-Khan, here sitting in Times Square pedestrian mall, plans to turn a full block of 34th Street into a similar mess.. Photo: Dan Brinzac

Be afraid — be very afraid — of what the Department of Transportation is up to on 34th Street.

The DOT’s detested bicycle lanes merely forced cars to park far from the curb and turned streets into parking lots. But the scheme that Commissioner Janette Sadik-Khan hopes will be her master stroke will ruin 34th Street from end to end.

Because of recent, overdue pushback against the bike paths and other Sadik-Khan moves, the DOT is advancing the $30 million project in secrecy, but remains hell-bent on making it happen — the public be damned.

The 34th Street initiative was announced last April — and apparently back-burnered soon after, to widespread relief. Yet it’s anything but shelved.

The DOT is still pushing to:

* Close the entire, long block between Fifth and Sixth avenues to vehicular traffic (except for buses) for a pedestrian mall,

* Create curbside bus lanes walled off from other traffic, and

* Make the street run one way in opposite directions from the mall — eastbound-only from Fifth Avenue, and westbound-only from Herald Square.

The plan would effectively turn 34th Street into a Manhattan exit causeway. Why not bring back Robert Moses’ cross-Manhattan expressway?

Publicly, DOT insists it’s all still up in the air. Asked about the status of the proposals, a spokesman offered this opaque response: “No decisions have yet been made as community discussion for this project continues. We are currently developing a detailed design (those presented previously have been conceptual) that will be presented to the community for feedback, with traffic study results expected later in the spring.”

Again, be afraid: The DOT has consistently ignored community objections, and its “studies” of its bike lanes’ and plazas’ impact on traffic have ranged from incomplete to outright false.

For a preview of how the block between Macy’s and the Empire State Building might soon look, check out Times Square’s asphalt-paved “plazas” cluttered with cheap chairs and tourists gobbling junk food Mayor Bloomberg claims to hate. The 34th Street plaza would be bigger than all of Times Square’s combined.

Sure, DOT is pretending to be “sensitive” to community concerns. It’s even preparing an environmental review — a concession it has not previously made. But it has to, to qualify for the federal funds that make the project possible. Even so, insiders call it a “review lite,” far short of the broader environmental-impact statements required of most large projects.

Meanwhile, the DOT is stealthily working to defuse minor objections by some landlords, business leaders and community representatives. It’s huddled with Vornado Realty Trust and the Malkin family, who fear the effect on deliveries and access. Its reps have also met with the 34th Street Partnership, Community Boards 4, 5 and 6, and civic groups including the Murray Hill Neighborhood Association.

But parochial concerns that the DOT might ameliorate — a crosswalk here, a delivery lane there — pale before the impact a wholesale remaking of 34th Street would have on the city.

In the name of relieving crosstown-traffic congestion, DOT would force cars and trucks to choose narrower streets north and south of 34th Street, with reckless disregard for consequences to one of the world’s great shopping and business districts.

One of New York’s most respected and successful executives, a venerated figure in the corridors of power, recently told me the DOT’s 34th Street plan was “insane,” but wouldn’t be quoted by name. Alarmed as they are by Sadik-Khan, business leaders are loath to jeopardize relations with City Hall — so none will speak up publicly.

Nor is there effective political oversight. The City Council, which must approve land-use proposals that depart an iota from zoning rules, has no authority over DOT. No other city agency has a meaningful say in DOT upheavals that irrevocably alter the urban flow and fabric.

Council Speaker Christine Quinn, Manhattan Borough President Scott Stringer and Rep. Carolyn Maloney have expressed reservations about 34th Street. They’ve asked the DOT to do a “block-by-block” analysis. That’s helpful, but doesn’t address the effect a redesign would have on all of Manhattan. It even plays into the DOT’s hands by suggesting that all the plan needs is minor tinkering.

What’s urgently needed is private-sector courage to denounce the whole, ill-advised scheme. David Letterman has ridiculed the “petting zoo” Sadik-Khan has made of the streets. But many more voices than a single entertainer’s are needed to demand that the agency not destroy one of Manhattan’s great boulevards to satisfy a mayor’s warped priorities and a commissioner’s unbridled ambition.